The present invention relates to power supplies for use with gas-discharge display lamps. More particularly, the present invention relates to power supplies for use with inert gas lamps such as neon lamps, for example, and lamps containing mercury and an inert gas.
Historically, in the early generations of neon signs, neon lamps or tubes forming the neon signs were powered with "core and coil" transformers operating at a low AC frequency such as the frequency of the public utility, for example. These transformers, however, were generally cumbersome to use because of their size and weight. It should be understood that the term "neon lamp" is used herein to refer to all gas-discharge lamps that use an inert gas and is not limited to lamps that contain only neon gas.
Later generations of neon lamps were powered with more compact power supplies operating at higher AC frequencies, typically in the kilohertz range and above.
One problem that occurs with the use of high frequency power supplies, however, is the generation of "bubbles" or "beads" in the gas discharge. The bubbles form a nodal pattern of alternating high and low intensity regions that resembles a string of beads. This nodal pattern is caused by standing waves that are present within the neon tube and which are produced by high frequency excitation of the gas. The pattern may move along the length of the neon tube depending on the excitation frequency of the power supply and the particular geometry or shape of the neon tube. In addition, the presence of bends and splices, for example, will affect the frequency at which bubbling occurs. Neon tubes are often formed into a complex assembly of letters or artistic shapes and designs, thus increasing the likelihood of bubbling. Therefore, it may not be technically feasible to select an operating frequency at which bubbling does not occur throughout the various neon tube lengths that are present in a complex neon tube assembly. In many cases, more than one nodal pattern will occur in a single neon tube, and each nodal pattern may move at different velocities and in different directions.
One way to eliminate bubbling is to add a DC component to the AC input power. FIG. 1 illustrates a conventional circuit for generating a DC component in a power supply for a neon tube. Voltage output from the high frequency AC voltage source 10 passes through the inductor 12, which limits the amount of current drawn by the neon tube 14. The input voltage is stepped up by an output transformer 16 to an appropriate level for driving the neon tube 14. An automatic bias circuit 18, consisting of a capacitor 20 and a diode 22 connected in parallel, allows current to flow in one direction from the anode 24 to the cathode 26 of the diode 22. Current flow in the opposite direction acts to back bias the diode 22, thus allowing the capacitor 20 to charge up and to produce the DC voltage component.
Mercury vapor is often used in neon tubes to alter the color of the light that is produced. Also, mercury vapor is commonly used in phosphor-coated neon tubes as a medium for exciting the phosphor to produce a luminous glow therefrom. Radiation produced in the mercury gas discharge is an effective excitation source for the phosphor coating.
Neon signs often have segments of different colors that are produced by using various phosphors and/or gases that discharge those different colors, and it is desirable to have a single power supply for the entire assembly.
When mercury-containing tubes are powered by a power supply having a DC component, such as that described above, mercury atoms tend to migrate toward the cathode or the negative end of the neon tube. This migration causes a deficiency of mercury near the positive end, which results in the undesirable effect of the negative end glowing brighter than the positive end. As discussed above, however, a DC component is necessary to prevent bubbling in neon tubes and therefore cannot be completely eliminated from power supplies used for mercury-containing lamps.
One method for reducing the effects of mercury migration is proposed in U.S. Pat. No. 5,189,343 and U.S. Pat. No. 5,367,225, both assigned to Everbrite, Inc. The Everbrite method consists of alternating the polarity of the DC current flowing through the neon tube by using high-voltage semiconductor switches connected to the secondary windings of the output transformer. An alternative method proposed by Everbrite is to apply an asymmetrical waveform to the neon tube, which acts in conjunction with the geometry of the neon tube to produce a DC offset current therethrough.
The generation of the DC current by use of an asymmetrical waveform may be understood by considering the voltage-current characteristics of a neon tube. When operated at a high frequency, the neon tube has voltage-current characteristics similar to a pair of Zener diodes D.sub.a, D.sub.b connected back to back and in series with a resistor R, as schematically shown in the equivalent circuit of FIG. 2. Little current will flow below the breakdown voltage of the diodes D.sub.a, D.sub.b, and above the breakdown voltage the current through the equivalent circuit, and thus through the neon lamp, is limited by the impedance of the external circuit connected thereto, such as by the impedance of the inductor 12 of FIG. 1. The resistor R of the equivalent circuit of FIG. 2 is not effective in limiting current because its impedance is, in general, low compared with the impedance of the inductor 12. Also, the neon tube can have bi-stable operating points, in which a single operating voltage can give rise to two different operating currents, and therefore the internal resistance of the neon tube (or R in FIG. 2) is not a predictable means for limiting current.
According to FIG. 3, if the high-frequency AC voltage source 30 has an asymmetrical output waveform, such as that shown in FIG. 4, a corresponding asymmetrical current is produced and supplied to the inductor 32 and then to the output transformer 36 of FIG. 3. This asymmetrical current flows from the secondary windings 38 of the output transformer 36 to the neon tube 34.
In theory, if the neon tube 34 is replaced with a purely resistive load, the asymmetrical current through the resistive load would resemble the waveform through the secondary windings 38. Specifically, as shown in FIG. 4, the average current over a complete current cycle would be equal to zero but the peak current would have a magnitude that depends on its polarity. In other words, the peak current during one polarity of the current cycle would be larger than the peak current during the other polarity of the current cycle, with the overall average current being zero over the complete current cycle.
In practice, the resistive load discussed above cannot adequately represent the neon tube 34 because the symmetrical nature of the neon tube 34 does not allow it to follow the asymmetrical current as faithfully as the resistive load would. Although the average voltage across the secondary windings 38 and the neon tube 34 is zero over a complete voltage cycle, the average current through the secondary windings 38 and the neon tube 34 is not zero. Instead, a DC offset current is established that acts to compensate for the asymmetrical current supplied to the neon tube 34. This DC offset current produced by the asymmetrical voltage source 30 serves to prevent bubble formation in the neon tube 34 in a manner similar to that in which the DC component produced by the automatic bias circuit 18 of FIG. 1 serves to prevent bubble formation.
An undesirable effect of establishing a DC offset current through the secondary windings 38 of the output transformer 36 is that the DC offset current can result in a DC offset flux produced by the transformer 36, which can result in premature core saturation. An air gap set up in the flux path may be used to prevent DC offset current-induced core saturation, however, such an air gap would lead to excessive losses in the transformer 36 due to stray flux emanating from the air gap.